Photoluminescent sensors or probes are a widely employed method of measuring analyte concentration, typically oxygen, within a defined space, typically an enclosed space such as the headspace of a package or container. See, for example United States Published Patent Applications 2009/0029402, 2008/8242870, 2008/215254, 2008/199360, 2008/190172, 2008/148817, 2008/146460, 2008/117418, 2008/0051646, and 2006/0002822, and U.S. Pat. Nos. 7,569,395, 7,534,615, 7,368,153, 7,138,270, 6,689,438, 5,718,842, 4,810,655, and 4,476,870.
Briefly, analyte concentration within a package or container can be measured by placing an analyte-sensitive photoluminescent probe within the package or container, allowing the probe to equilibrate within the package or container, exciting the probe with radiant energy, and measuring the extent to which radiant energy emitted by the excited probe is quenched by the presence of the target analyte. Such optical sensors are available from a number of suppliers, including PreSens Precision Sensing, GmbH of Regensburg, Germany, Oxysense of Dallas, Tex., United States, and Mocon, Inc. of Minneapolis, Minn., United States.
In order to permit impromptu testing of a defined space, the photoluminescent probe can be provided as a coating on the distal tip of a fiber optic filament which is threaded into the lumen of a needle and protectively retained in a fixed position within the lumen by a target analyte permeable encapsulant. One example of such a fiber optic sensing needle for use in measuring the concentration of oxygen within living tissue is described in United States Patent Application Publication US 2009/0075321, the entire disclosure of which is hereby incorporated by reference.
While fiber optic sensing needles, such as that described in US 2009/0075321, are effective for impromptu measurement of analyte concentration within a defined space, they can result in inaccurate measurements when the total pressure within the defined space is significantly different than the environment surrounding the defined spaced as the fiber optic sensing needles measure analyte partial pressure—not concentration of analyte, from which a concentration of analyte is then calculated using the general equation ANALYTE %=PANALYTE/PTOTAL. Hence, lack of an accurate assumption as to the total pressure PTOTAL of the defined space can result in an inaccurately calculated analyte concentration.
Hence, a need exists for a fast response fiber optic sensing needle that can quickly and easily measure both analyte partial pressure PANALYTE and total pressure PTOTAL in a defined space and thereby consistently provide a pressure correct measurement of analyte concentration (ANALYTE %).